Visual television transmitter



OCII. 13, 1959 ERNYEl ETAL 2,908,753

VISUAL TELEVISION TRANSMITTER Filed Jan. 24, 1955 5 Sheets-Sheet 1 Rerence 5 nc Back Picture -B)Ankin Ref f h lglach alse unch signal levelwhlte Transmitter Oct. 13, 1959 H. ERNYEI ETAL VISUAL TELEVISIONTRANSMITTER 3 Sheets-Sheet 2 Filed Jan. 24, 1955 Amplitude Modulator e H.rL [I a a e E w 2 ct 2 mm VR m 0 am A 2 O .r I a m n a ol DI m w mw l mV A C DCBias FlG.2

0a. 13, 1959 I H, ERNYE, ETAL' 2,908,753

VISUAL TELEVISION TRANSMITTER Filed Jan. 24, 1955 5 Sheets-Sheet 3 E A.U

towards video Fry. 3 channel United States Patent VISUAL TELEVISIONTRANSMITTER Herbert Ernyei and Paul Simonrl-Cote,v Paris, France,assignors to Societe Nouvelle de IOutillage 'R.B.V. et de laRadio-Industrie, Paris, France Application January 24, 1955, Serial No.483,668 Claims priority, application FranceJanuary 23, 1954 6 Claims.(Cl. 1'787.1)

The present invention is related to visual television transmitter andmore particularly to an arrangement for stablizing the blanking level inthe radiated television signal. As is Well known, the radiated visualtelevision signal is submitted to government regulations. The modulatedtelevision carrier wave is shown on Figure 1 curve A. It consists mainlyin video informations such as shown at 1, 2, 3, etc., which modulatenegatively the amplitude of the carrier wave. This means that Whitespots of the picture correspond to the lowest carrier amplitude, whileblack spots correspond to the higher carrier amplitude. shown on thepicture as reference black level 4. This reference black level shouldcorrespond to 60% of the peak power of the transmitter. The radiatedsignal comprises also synchronizing pulses such as 5, 6, etc., which areadded to a reference carrier level .known as blanking level, shown as 7on curve 1 Before and after the synchronizing pulses, the video signalshows two constant level porches during which the signal amplitudeisequalto the blanking. level. Government regulations specify also thatthe blanking level shall always correspond to a specific power levelregardless of the changes in the picture brightness. Owing to the factthat the amplitude of the synchronizing pulses is also regulated, thismeans that the tips of the pulses correspond to a constant level duringthe transmission. This constant level is shown at 8 on cure 1 andcorresponds nearly to the peak power of the transmitter.

The process of bringing all the synchronizing signal peaks to a commonreference level amounts to reinserting the. DC. component in-theradiated signal. To obtain a Qorrectradiated signal, it is necessarythat the carrier wave should be absolutely free of anyamplitudemodulation such as may result from hum or any other faulty Operationeither of the video circuit or of the carrier frequency channel. It isactually very difficult to obtain a modulated television carrier waveabsolutely free of any spurious signal and especially of humming. Curve1;; of Figure 1 shows the same modulated carrier wave to which is addeda slight amplitude modulation at sixty cycles (hum). To make thismodulation visible, it is necessary to use a different time scale fromthat of curve 1 owing to the very slowamplitude modulation correspondingto hum. After rectification of the radiated signal such as shown on cure1 the video signal which is obtained is no longer in accordance with theregulations since the tip synchronizing pulse level 8, the blankinglevel 7, and the black level 4 are no longer constant. These levels areamplitude modulated according to the spurious 60 cycles signal. Thisamplitude modulation of the carrier wave may be compensated by negativefeedback, by means of a signal obtainedxby peak detection of theradiated signal, said negative feedback controlling either the carrierchannel or the video channel after D.C. reinsertion. However, thisamplitude modulation of the carrier wave may produce a correspondingamplitude distortion of the synchronizing pulses which correspond to awrong ration of video signal to synchro Patented Oct. 13, 1959 sible touse negative feedback to control the ratio of video amplitude tosynchronizing pulse amplitude at a stabilizing amplifier. Owing to thefact that the peak power of the transmitter is constant, a compressionof the video signal will appear at the white levels which is mostdetrimental to the contrast rendition of the picture and to its quality.

The use of negative feedback does not provide means to compensate forwhite level compression. On the other hand, such a negative feedback isalways rather difficult to establish owing to the very low frequency ofthe control signals. T o obtain an accurate feedback, it is necessary toprovide a feedback loop in which all the voltage supplies are perfectlywell stabilized. This leads to cumbersome and expensive networks. Toprovide for a correct control of the feedback, itwould be necessary thatthe control signals should include the DC. component, that is anabsolute measurement of the black level with respect to zero carrieramplitude. Transmission of DC. signal through any series of networkleads to wellknown complications.

Accordingly, it is an object of the invention to provide levelstabilizing means whereby correct transmission of a video signal isobtained without the need of a DC. transmitting feedback network.

It is another object of the invention to provide level stabilizing meanswhereby correct transmission of the video signal may be obtained inspite of supply voltage variations of 10% in the carrier wave channeland at the modulator.

It is another object of the invention to provide level stabilizing meanswhereby correct transmission of a television signal may be obtainedwithout stabilisation of the supplies of the carrier channel of thetransmitter (carrier generator, modulator and amplifiers).

According to the main feature of the invention a compensation of thecarrier wave amplitude modulation is provided by negative feedback bymeans of aseries of control'pulses, amplitude modulated by anil or zeromethod according to the slow modulation of the blanking level in theradiated television signal.

According to a preferred embodiment of the invention the frequency ofthe control pulses is constant and equal to the line scanning frequency;the duration of the pulses is longer than the duration of the back porchof the line blanking pulses used with the same video signals.

The invention will be understood by reference to the followingdescription and the accompanying drawings drawings among which Figue 2is a block diagramm of a television transmitter and Figure 3 is thewiring diagram of the feedback control circuit for amplitudestabilization of the carrier wave.

Figure 2 shows the power carrier frequency amplifier 20 which feedsaerial 21 directly or through a side band rejection filter not shown.The radiated signal is rectified by circuit 22 of the feedback channel.The output from circuit 22 reproduces the modulation of the carrierwave. It corresponds either to curve 1 or 1 of Figure l. Rectifiedsignal is transmitted by means of gate 23 to the measurement network 24.Gate 23 is opened during the blanking back porch such as A, B of Figurel by means of control pulses shown on curve I of Figure l. Measurementnetwork 24 delivers amplitude modulated control pulses which aredirectly used as control signalina D.C. reinserting network 25 of stage26 of the'video channel of the transmitter. It will be supposed thatcoupling between the video stage 26 and the carrier frequency modulator27 is 'a- DC. transmitting coupling;

- by the positive pulse applied to grid G of V a The only elements whichare used for the feedback control are network 22, 23 and 24 placed inthe interrupted line rectangle 28. The other circuits, which have justbeen referred to, belong to the transmitter. The D.C. reinsertingnetwork 25 consists in a bidirectional clamper controlled by means ofclamping pulses in phase but slightly shorter than control pulses A, Bof curve 1 of Figure 1. Such a circuit is fully described for instancein U.S. Patent to Karl R. Wendt No. 2,299,945.

Figure 3 is a detailed wiring diagram of the circuits which are includedin rectangle 28. The carrier frequency radiated television signal isrectified in the video rectifier 22 which comprises an ordinary vacuumdiode or a semi-conductor diode. The output video signal is transmittedto the control grid G of the left hand side of a double triode V shownas V Stage V is the measuring network 24 of Figure 2. The left handelement V of tube V; is normally cut-off owing to the positive biasvoltage applied to its cathode by resistor R in which flows the anodiccurrent of tube V which is normally conducting. Tube V constitutes withtube V and V the electronic gate 23 which will make stage V conductingduring time intervals A, B of Figure 1D. Actually negatively polarisedpulses with the correct duration and phasing are delivered by a flipflop circuit comprising tubes V and V so as to cut-off V during timeintervals A, B. When V is cut-01f, the cathode potential of the lefthand element V is equal to earth potential and the tube becomesconducting. The operation of gate circuit 23 is as follows. Linesynchronizing signals, delivered by the line synchronizing generator,are applied at input E and transmitted by means of the diiferentiatingcircuit C R to control grid G of the left hand element of tube V Diode Dwhich is connected in shunt with resistor R is intended to short circuitnegative pulses resulting from the ditferentiation of the leading edgeof the pulsed synchronizing signal applied at E input. Both elements oftube V are interconnected as a Schmitt trigger circuit such as describedin the Journal of Scientific Instrument, 1938 XV, page 24, ThermionicTrigger, controlled The duration of the positive pulse delivered acrossload impedance of V tube is determined by the time constant of thecircuits associated to V and V and especially to the time constant C RIt is chosen longer than the actual duration of the line blanking backporch. However, since gate 23 is triggered by the lagging edge of thesynchronizing pulses, the leading edge of these gating pulses is inphase with usual clamping pulses.

The output pulses from gate 23 are shown on curve I The polarity of theoutput pulse from trigger V is inverted in amplifier V The negativepulse output from V has sufiicient amplitude to cut-off V Themeasurement network 24 comprises stage V connected in shunt with elementV The steady bias voltages on this tube are preset so that the tube isnormally conducting. The output current from V is such that when V iscut off, the normal output current from V is equal to the current whichflows through V when this element is conducting and receives a signalthe amplitude of which corresponds to nominal black level. In otherwords the output current through V is equal to the current which wouldflow through V if the amplitude of the back porch of the video signaldelivered by diode 22 were correct. That is to say if there were noamplitude modulation on the carrier wave. The gating pulses of curve 1delivered by amplifier V are simultaneously applied to the grid G oftube V so as to cut-off this normally conducting tube and to the controlgrid G of right hand element V so as to cut-off this tube also.Accordingly, left element V is conducting during pulses of curve I andthe output current of V stage is modulated according to the amplitude ofthe video signal delivered by diode 22 during time interval A, B.Normally, tube V is-blocked and tube V conductive. When the gate pulsesfrom 23 are applied to the grid of V this tube is blocked and thevoltage appearing at terminals of resistor R corresponds to that of thenominal black level chosen as a reference as described above and gatepulses from 23 are applied to the grid of tube V so that this tube isblocked and, accordingly, tube V conducts. This tube V receives on itscontrol grid the radiated signal from 22 during the time determined bythe gate pulses of 23. According to the amplitude of this signal, thecurrent flowing through resistor R causes the voltage existing at theterminals of this resistor to drop so that the resulting voltage appearsas the difference between the nominal black level and the black level ofthe radiated signal and may be either positive or negative. The outputvoltage across load resistor R is proportional to the difference in thecurrents flowing respectively through V and V This output voltageappears as a pulsed voltage of duration equal to time interval A, B,which is either positive or negative. The absolute value of this pulseis directly related to the difference of the currents flowing througheach of the elements of tube V This pulse constitutes a measurement ofthe actual black level by reference to an arbitrarily preset level whichis fixed by the values of the DC. bias voltages on tube V The controlpulse across resistor R is transmitted by means of coupling condenser Cto stage V which is only a outphasing circuit and which feeds thecathode follower stage V The load P of voltagev V is a potentiometer themoving arm of which is connected to the DC. reinserting network 25 ofthe video channel amplifier 26. The efiiciency of the feedback controlis adjustable by moving the arm of P6.

What we claim is:

1. In a television system having a video transmitter channel, anarrangement for stabilizing the blanking level of the transmitted radiofrequency signal comprising, means controlled by the radiated videosignal for producing control pulses repeated at the frequency of thesynchronizing pulses means to amplitude modulate said control pulsesaccording to the absolute value of the difference between a nominal orreference black level and the black level of the radiated carrier wave,and means for feeding-back said control pulses to said video channel.

2. In a television system having a video transmitter channel, anarrangement for stabilizing the blanking level of the transmitted signalcomprising, means controlled by the radiated signal for producingcontrol pulses having their leading edges synchronized with the rearedges of the line synchronizing pulses and of a duration longer than theback porch of the television signal, means for modulating the amplitudeof said control pulses in ac cordance with variations in the amplitudeof the radiated carrier wave to obtain difference control pulses, andmeans for supplying said difference control pulses to the video channel.

3. In a television system having clamping means in a video transmitterchannel, an arrangement for stabilizing the blanking level of thetransmitted signal comprising, means controlled by the radiated videosignal for producing control pulses in synchronism with the clampingpulses applied to said clamping means, said control pulses beingamplitude modulated according to variations in the amplitude of theradiated carrier wave to obtain differ ence control pulses, and meansfor supplying said dif* ference control pulses to said clamping means.

4. In a television system having clamping means in a video transmitterchannel, an arrangement for stabilizing the blanking level of thetransmitted signal comprising, means controlled by the transmittedsynchronizing pulses for producing gating pulses synchronized with theclamping pulses applied to said clamping means, gating means controlledjointly by said gating pulses and by the radiatted televison signal forproducing control pulses in synchronism with said gating pulses and ofan amplitude dependent upon the difference between the amplitude of theactual black level of the television carrier wave and a nominal orreference black level, and means for supplying said control pulses tosaid clamping means.

5. In a television system having D.C. reinsertion means in a videotransmitter channel, an arrangement for stabilizing the blanking levelof the transmitted signal comprising, means controlled by thetransmitted synchronizing pulses for producing gating pulsessynchronized with and of longer duration than the back porch intervalsof the television signal, detector means rectifying the transmittedcarrier wave and producing a modulating signal varying in accordancewith amplitude variations of said carrier wave, a balanced modulatorcontrolled jointly by said gating pulses and by said modulating signalfor producing control pulses in synchronism with said gating pulses andof an amplitude and polarity dependent upon direction and extent ofchange of the amplitude of said modulating signal from a given value,and means for supplying said control pulses to said D.C. reinsertionmeans. 7

6. In a television system for transmitting a video modulated carrier andhaving a video amplifier, a modulator References Cited in the file ofthis patent UNITED STATES PATENTS 2,190,753 Browne Feb. 20, 19402,295,330 Blumlein Sept. 8, 1942 2,307,375 Blumlein Jan. 5, 19432,445,040 Schade July 13, 1948 2,706,220 Schlesinger Apr. 12, 1955FOREIGN PATENTS 514,993 Belgium July 23, 1954

